シンプルなプリローダー

preloader.ts

/** Single preloader */
export const preload =
  (url: string): Promise<string> =>
    fetch(url).then(res => res.blob()).then(blob => URL.createObjectURL(blob))

/** Multiple preloader */
export const preloadAll =
  (urls: { [key: string]: string }): Promise<{ [key: string]: string }> =>
    Promise.all(Object.entries(urls).map(async ([k, v]) => [k, await preload(v)])).then(Object.fromEntries)